<!DOCTYPE html>
<html>
<body>

<select id="a" size="5">
<option>Option 1</option>
<option>Option 2</option>
<option>Option 3</option>
<option>Option 4</option>
</select>

<script>
document.body.offsetWidth;
var x=document.getElementById('a').options;
x[1].defaultSelected = true;
x[2].defaultSelected = true;
x[3].defaultSelected = true;
</script>
</body>
</html>
